Patterning of dense gratings with sub-wavelength pitches presents a challenge that can be addressed using Resolution Enhancement Techniques (RETs) such as dipole illumination, with the dipole axis perpendicular to the dense line orientation. However, this approach leads to pitch and orientation limitations that must be accommodated in layout practices and design rules. In this work we evaluate the impact that dipole illumination has on the process window of isolated lines and loose pitch lines parallel and orthogonal to the dipole axis, and demonstrate the use of OPC and design restrictions to minimize this impact. Semi-dense and isolated features need to be treated as a function of their orientation with respect to the dipole. Specifically, isolated features oriented along the axis of the dipole have larger process margins than the same feature oriented perpendicular to this axis. We systematically explore the process margins for various CDs, pitches and orientations, and compare the results with simulations. We demonstrate that the dipole illumination restricts the ranges of sizes, pitches and orientations that can be printed with sufficient process margin. Knowledge of these restrictions and comparing them with simulation enables us to evaluate the suitability of simulations as a predictor for design rules to restrict layout. The results enable us to propose design rules that would enable single-mask solutions for layers using dipole illumination.